Malignant plasma cells secrete a number of effector and regulator products; among these are immunoglobulin, osteoclast activating factor, interferon and PC-factor. PC-factor appears to act upon macrophages, inducing them to secrete a plasmacytoma-induced macrophage substance (PIMS) which suppresses antibody production by antigen-stimulated normal spleen cells. This regulatory loop is responsible for the profound immunosuppression observed in hosts of plasma cell tumors. PC-factor is produced and secreted into the surrounding medium by malignant plasma cells. Studies are proposed to isolate, purify and characterize the PC-factor. Studies will be conducted to determine the type of macrophages which are stimulated by PC-factor to produce PIMS and to determine the type of macrophages which are stimulated by PC-factor to produce PIMS and to determine if the signal transmitted by PC-factor can be mimicked by any other tumor-produced factors or macrophage stimulating agents. Experiments will also be performed to determine if PC-factor is produced by normal, non-malignant plasma cells or by mitogen- or antigen-stimulated B lymphocytes.